November 2017 Archives

Forming a thick layer of plating, such as industrial nickel plating or electroformed nickel coatings, will leave stress in electrodeposits.The stress in electrodeposits comes in two forms: compression stress and tensile stress. If uncontrolled, they will lead to adhesion failures or deformation in sizes and shapes over time.

(1)Measurement method of stress in electrodeposits

The available methods are listed below.Since it is not possible to measure the actual stress in electrodeposits on the products, the stress will be calculated by using a dedicated test piece that will be plated or electroformed in the same conditions.

(2)Calculation of average stress in electrodeposits

After applying plating on a rectangular test piece with insulation coating applied only one side (mild steel plate, 0.1 to 0.3mm thick, 10 to 15mm wide, and 50 to 100mm plating area), measure the force required to restore the deformation in the longitudinal direction, which was caused by electrodeposition stress, and calculate the average. [Fig.1] is the illustration of a measuring device.

(3)Measurement by spiral contract meter

Prepare a rectangular test piece (mild steel, 20 mm wide, and 0.2 to 0.5 mm thick) and wind it diagonally around the 20 mmΦ iron core, spaced approximately 0.5 mm apart. This will make a spiral (tabular spring) test piece (a). After insulating the internal surface, secure the top of the test piece onto the fixed axis of the top of the contract meter (b), while securing the bottom of the test piece onto the rotating shaft at the lower end. Apply plating over this test piece in the same conditions as those for the actual product.Using the gear mechanism, the dial gauge displays the angle of the twisted spiral that was caused by the electrodeposition stress. [Fig.2] shows a structure of the device.

(4)Reduction of stress in electrodeposits

The stress in electrodeposits can be reduced drastically by the plating conditions and using plating additives for stress reduction where appropriate. However, even these measures combined, it will not cover the entire surfaces.Heat treatment is generally adopted for reducing the stress in products. The conditions will be determined and agreed between the parties concerned.

The chromate treatment over zinc plating is designed to add gloss finish or color tones, such as pale yellow, yellow, and green, while drastically enhancing the corrosion resistance by forming anti-corrosive films over the zinc-plated surfaces.

(1) Grades, types, and symbols of chromate films

[Table 1] Grades, types, and symbols of chromate films

(2) Mass measurement method for chromate films

For the measurement, it is necessary to prepare a test piece consisting of the surface area shown in [Table 2] (24 hours but no more than 14 days elapsed from the chromate treatment), and the one-liter solution that is made by dissolving 50 g of sodium cyanide and 5 g of sodium hydroxide into pure water. The mass is measured by a reduction of weight after soaking the test piece into this solution for one minute, while cathode current density stays at 15 A/dm2 to dissolve the chromate film.

[Table 2] Total surface area of test pieces

(3) Corrosion resistance of chromate films

The minimum hours shown in [Table 3] must be satisfied after conducting a neutral salt spray test consecutively.

[Table 3] Corrosion resistance of chromate films

(4) Notation of chromate films

Ep-Fe/Zn 25/CM2 D

Electro-galvanizing deposited at 25 µm or higher on a steel substrate, topped with Grade 2 green chromate film

Ep-Fe/Zn 8/CM1A

Electro-galvanizing deposited at 8 µm or higher on a steel substrate, topped with Grade 1 gloss chromate film

JIS defines the requirements on plating applied over iron, copper, or their alloy substrates, in order to add solder wettability, electrical properties, and corrosion resistance to electrical/electronic components, as described hereinafter.

(1)Type and grade of plating

[Table 1] Chemical composition (%) of plating film

[Table 2] Type and grade of plating

(2)Appearance of plating

The visual inspection will be performed in the same manner as the other types of plating. The plating surface should be flat and free from defects, including burnt or swollen areas, lumps, scratches, pits, flaking, cracks, and exposure of substrates or undercoat.

(3)Chemical composition of plating film

The chemical composition of plating film can be measured by the fluorescence X-ray analysis, and so on.

(4)Minimum thickness of plating film

The plating thickness can be measured by instruments, such as a fluorescent X-ray film thickness meter and a magnetic film thickness meter. In both cases, the values shown in Table 2 must be satisfied.

(5)Plating adhesion

It should be measured by conducting an adhesion-tape test, a bending test, a thermal-shock test, and so on.

(6)Corrosion resistance of plating

The corrosion resistance must be verified to be at least rating number 9 by conducting a neutral salt spray test or a test method agreed between the parties concerned.

(7)Solder wettability of plating

Just like the case of silver coatings applied for engineering purposes, the following solder wettability test must be conducted:Immerse the flux-treated sample into a soldering bath heated at 250°C for three seconds, and shake the residual liquid off. Check the soldering surface and evaluate the property.

(8)Stress relief before plating

To include a heat treatment process for reducing residual stress on steel/copper alloy products before plating, the conditions must be determined and agreed by the parties concerned.

(9)Plating name and symbol

●Ep-Cu/Ni/Sn(75)-Pb5

Tin (75%)-lead alloy plating with a thickness of 5 µm over nickel base-coat plating on a copper substrate

●Ep-Cu/Ni/Sn(75)-Pb[3]

Type I/Grade 3 tin-lead alloy plating deposited over nickel base-coat plating on a copper substrate

●Ep-Fe/Cu/Sn(75)-Pb5

Tin (75%)-lead alloy plating with a thickness of 5µm over copper base-coat plating on an iron substrate

JIS defines the requirements on silver plating that is 0.5 µm and thicker for decorative purposes as described hereinafter. The decorative purposes mentioned here are designed for usages with tableware, personal ornaments and miscellaneous goods in which appearance, corrosion resistance, and abrasion resistance are valued the most.

(1) Appearance

The significant surfaces of the plating should be free from defects of polish, radiance, color shades, and imperfections as roughness, burnt parts, cracks, pits, dendrites, plating defects as substrate or undercoat exposure, adhesion failure as swelling or external flaws including smudges and scratches.

(2) Plating thickness

The plating thickness will be determined and agreed between the parties concerned. The thickness must meet the specified value at any point on the significant surfaces. [Table] shows the minimum plating thicknesses that are commonly used.

[Table] Classification by the minimum plating thickness

(3) Porosity of plating

If the porosity of plating is specified, it must pass the porosity test as agreed.(See the corresponding section in "Electroplated Coatings of Silver for Engineering Purposes.")

(4) Color fastness of plating

If the color fastness is specified for anti-tarnish treated plating, the property must be evaluated by measuring the time until a discoloration occurs after dipping the sample into the ammonium sulfide solution.

(5) Plating adhesion

The plating adhesion can be verified by a tape test or a bending test.

(6) Plating hardness

The plating section or surface must be measured by instruments, such as a micro Vickers hardness meter.

(7) Abrasion resistance of plating

It should be determined by methods defined in the JIS abrasion test method or those agreed between the parties concerned.

(8) Base-coat plating

If the parties concerned agreed to have base-coat plating applied for improving appearance, corrosion resistance, or adhesion properties, the plating type and thickness must also be specified.

(9) Plating name and symbol

Ep-Cu/Ni 5 b,D-Ag 5

Decorative silver plating with a thickness of at least 5 µm over bright nickel base-coat plating on a brass substrate

Ep-Cu/D-Ag 10 / AT

Decorative silver plating of 10 µm or thicker deposited on a brass substrate, treated with anti-tarnish chromate finish

EP-Zn/Cu 10,Ni 12 b,D-Ag 5

Decorative silver plating with a thickness of at least 5 µm over copper base-coat plating with a thickness of 10 µm and bright nickel base-coat plating with a thickness of 12 µm on a zinc die casting alloy substrate